I currently have a iPhone 4 and I am upgrading to an iPhone 4S. My sister does not have a smartphone and her line is ready for an upgrade, but my line is not. The plan is to give her my old iPhone 4 once I get the Iphone 4S using her upgrade. However, we both want to keep our same numbers. Would it work out if we bought and registered the new iPhone 4S on my sister's line, and then just switched the sim cards? Thanks!